The threshold of perception of field flicker in a television display system is a function of the flicker frequency and the brightness of the display. Over the years displays have increased in brightness to the point where flicker is noticeable even in relatively high field rate systems (e.g., the NTSC 60 Hz system) and clearly objectionable in lower field rate systems (e.g., the PAL 50 Hz system). A solution to this problem is to double the field rate of displayed images as described, for example, by Lord et al. in U.S. Pat. No. 4,322,750 entitled TELEVISION DISPLAY SYSTEM which issued Mar. 30, 1982. In an example of the Lord et al. system, a video input signal is stored in a field memory. Each stored field is recovered or "read" twice from the memory and displayed on a display scanned at double the line rate and double the field rate of the incoming video signal thereby doubling the flicker frequency of displayed images and thus reducing the visibility of flicker.
Lord et al. recognize that doubling the field rate of an interlaced signal by repeating fields results in a non-standard field sequence of even-even-odd-odd (E-E-O-O). In order to preserve the vertical resolution of displayed images it is important that even fields overly even fields, that odd fields overlay odd fields and that even and odd fields be interlaced when the double field rate signal is displayed. Lord et al. propose that a E-E-O-O display interlace pattern may be implemented by "perturbing" (i.e., offsetting) the display horizontal scan waveform by one-half line every other field. As an alternative, Lord et al. suggest that the vertical scan waveform may be "perturbed". The specific waveform proposed by Lord et al. for "perturbing" the horizontal scan signal requires half-line scans at the end of first and third fields of a four-field sequence with the half-line scans having different reset levels. Specifically, the horizontal scan is reset to its starting point during the last half-line of the first field of the sequence and is reset to its midpoint level for the last half line of the third field of the sequence. A scanning pattern having different reset levels is difficult to implement and particularly so in a high field rate system.
Lord et al. do not disclose any specific means for "perturbing" the horizontal or vertical scan waveforms. It is known generally, however, that display interlace patterns may be altered by adding an offset current to the yoke of a kinescope as described, for example, by Loe in U.S. Pat. No. 2,681,383 which issued June 15, 1954. The use of deflection offset currents for interlace control has certain disadvantages, however, particularly as applied to a field repeating flicker reduction systems. In flicker reduction systems the field rate is relatively high (e.g., 100 Hz for PAL or 120 Hz for NTSC). This high field rate places more critical requirements on the deflection circuit components and complicates the design of the offset current source. Special compensation circuits for the offset signal may be required to minimize interlace errors due to high voltage variations and to avoid line-pairing, miss-convergence, pin cushion problems and other undesirable visual artifacts.